1. Field of the Invention
This invention relates to a thermal printer for coloring thermosensible paper by means of Joule heat produced by heating resistor elements.
2. Description of the Related Art
One known printing device contains a thermal printer colors thermosensible paper by means of Joule heat produced by heating resistor elements, and comes in two varieties one is known as a line printer which prints in a line-by-line fashion, while the other, known as a dot printer, which prints in a dot-by-dot fashion for each line of print.
In the thermal printer, current is fed and not fed to each heating resistor element in accordance with the printed data "1" and "0". When current is fed to the heating resistor element, the element generates Joule heat as given by EQU (V.sup.2 /R).multidot..DELTA.t (.perspectiveto.3mJ)
where
R: the resistance of the heating resistor element, PA1 V: the voltage supplied, PA1 .DELTA.t: current feed time period.
This Joule heat colors a thermosensible paper, resulting in printing.
Since the current feed period of a thermal printer using such heating resistor elements if fixed, this results in the density of color not being uniform over all of the printed paper. The heating resistor element generates a fixed amount of Joule heat during a current-feed period, and when the current feed period terminates, dissipates a fixed amount of Joule heat. A surface temperature of the thermal head including the heating resistor elements gradually rises during the current feed period, and gradually falls when the current feed period terminates. Because of the fixed current feed period for the heating resistor elements, when "1" data is successively printed, the printing of the next "1" data starts before the head surface temperature completely drops to the initial temperature. Accordingly, the peak value of the head surface temperature gradually rises, and a color density on the thermosensible paper becomes high Consequently, color densities of dots are not uniform over the paper.
To solve this problem, there is a proposal of a thermal printer as disclosed in KOKAI No. 55-160388. In the proposed printer, the record data applied to the thermal printer is detected, and the next drive pulse of the thermal head is chopped according to a particular detection result. More specifically, if the previous data is "1", the present data is printed in such a way that during a fixed current feed period, the chopped pulse is applied to the thermal head, that is, the actual current feed duration is shorter than the fixed current feed duration. The proposal requires a high frequency clock circuit for generating high frequency drive pulses. Further, during a period that the chopped drive pulse is applied to the head, the previous record data as well as the present record data is also retained. The proposal, therefore, is not applicable for such a printer that when the present data is being printed, accepts the next record data.
The above Gazette also teaches a technique that the record data applied to the thermal head is detected, and a voltage of the next drive pulse for the thermal head is varied on the basis of the detection result Specifically, when the previous record data is "1", the present record data is printed while lowering a voltage of the drive pulse applied to the thermal head, with the intention to lower the heating temperature of the head. It is noted, however, that to change the voltage is to change the current, and this changes various parameters. The change of the parameters makes it difficult to control the current feed to the head.